you need soul powder correct? if so just do what i did and wipe the mobs clean in the necrohol of nabudis, youll have the chance at getting the soul of thamasas from oversouls and also after you kill the 6th oversoul youll fight helvnik which has a chance to drop a high arcana. I probably cleared that place like 15 times and got maybe like 4 soul of thamasa and 1 or 2 high arcanas. worked nice.